1. Field of the Invention
The present invention relates to a current controlled switching mode power supply, and more particularly, to an apparatus for adjusting a turn on/off time of a switching device by controlling a leading edge blanking (LEB) time and an external drive current of the switching device by means of a switching controller, thus capable of preventing a switching current from being excessive due to a delay of a turn-off time of the switching device, which is caused by a circuit delay during a soft start of the switching mode power supply. Also, the present invention can prevent a switching current from being excessive due to a failure in accurately controlling a turn-off time of the switching device because of a delay caused when an output voltage (a voltage at a secondary winding of a transformer) of the switching mode power supply is designed to have a high voltage.
2. Description of Related Art
FIG. 1 is a schematic circuit diagram of a conventional switching mode power supply (SMPS) 100. A conventional switching mode power supply includes a direct current (DC) voltage supply 110, a voltage output block 120, feedback circuit blocks 130a-b, and a switching controller 140. The DC voltage supply 110 includes a bridge diode rectifier 111 and a capacitor (C1) 112. The voltage output block 120 includes a transformer 121, a diode (D1) 122, capacitors (C2, C5) 124 and 125, and an inductor 123. The transformer 121 has a primary winding Lp and a secondary winding Ls. The feedback circuit block 130b includes an amplifier (not shown), a photo coupler 131, and a capacitor (C4) 132. The switching controller 140 includes a source/sink unit 141, a pulse width modulation (PWM) unit 142, an oscillator 144, a leading edge blanking (LEB) unit 145, a switching device 147, a switching device sensing resistor 148, a soft start unit 149, and a protection unit 150. The PWM unit 142 includes a PWM comparator 143, NOR gates 151 and 153, a latch circuit unit 152, and a driver 146. The switching device 147 may be a metal oxide semiconductor field effect transistor (MOSFET). Hereinafter, the switching device is also referred to as a MOSFET 147 and the switching device sensing resistor 148 is also referred to as a MOSFET sensing resistor 148. However, other types of switching devices, such as bipolar junction transistors, are also within the scope of the described SMPS.
An operation of the conventional switching mode power supply will be described below. When an alternating (AC) voltage is applied to the DC voltage supply 110, the bridge diode rectifier 111 rectifies the AC voltage to produce a DC voltage, which is smoothed by the capacitor (C1) 112. The DC voltage charges a capacitor C3. Voltage supply 110 provides supply voltage Vcc to the switching controller 140 at the “start” pin, coupled in parallel to capacitor C1. When the voltage of capacitor C1 increases, the supply voltage Vcc also increases. The switching controller 140 begins to be driven when the capacitor C1 is charged to a suitable predetermined level. During an initial operation of the switching mode power supply 100, the PWM unit 142 receives a soft start voltage (Vsoft) and a MOSFET sensing voltage (Vsense): the voltage across the MOSFET sensing resistor 148. During a normal operation after the start up operation, the PWM unit 142 receives a feedback voltage (Vfb) through source/sink 141 and the MOSFET sensing voltage (Vsense), and outputs pulses having a predetermined duty ratio to a gate terminal of the switching device 147. The switching device 147 repeats on/off operations in response to the pulses. Since the switching device 147 is in an off state when the switching mode power supply is initially driven, the smoothed DC current Ip does not flow through the primary winding Lp of the transformer 121. When the PWM unit 142 is in a state that it turns on the gate terminal of the switching device 147, the smoothed DC current Ip flows through the primary winding Lp of the transformer 121, building up the primary energy Ep stored in Lp, since the primary energy Ep is proportional to (½) Lp×Ip2. When the switching device 147 is in an off state, the smoothed DC current Ip cannot continue flowing through the primary winding Lp, and the primary energy Ep stored in the primary winding Lp is transferred to secondary energy Es of the secondary winding Ls of the transformer 121. The corresponding induced secondary current Is of the secondary winding is rectified to a positive current by the diode (D1) 122 and is smoothed by the capacitor (C2) 124. The corresponding smoothed voltage Vs across the capacitor (C2) 124 modified by the capacitor (C5) 125 and inductor 123 becomes the output voltage Vout of the switching mode power supply 100. The switching controller 140 decreases the duty ratio of the clock pulse, outputted by oscillator 144 when the voltage across the capacitor (C4) 132 increases, while the switching controller 140 increases the duty ratio of the outputted clock pulse when the voltage across the capacitor (C4) 132 decreases. When the duty ratio of the clock pulse increases, the transferred secondary energy Es and induced secondary current Is increase. When the duty ratio of the clock pulse decreases, the transferred secondary energy Es and induced secondary current Is decrease. The secondary current of the secondary winding is adjusted by the switching operation of the switching device 147. Adjusting the secondary current adjusts the magnitude of the output voltage of the switching mode power supply 100.
The LEB unit 145 of the switching controller 140 is an element for controlling a surge current that is generated during the initial operation of the switching device 147. That is, when the switching device 147 changes from the initial off state to the on state, a surge current is generated. In the time interval when the surge current is generated, the LEB unit 145 performs a window (control) function that makes the PWM unit 142 not change the switching device 147 to the off state.
The amplifier (not shown) of the feedback circuit block 130 amplifies the output voltage Vout to a predetermined level at which the photo coupler 131 is enabled to operate. When the amplified output voltage rises above a predetermined level, the photo coupler 131 operates to discharge the capacitor (C4) 132 through the current source/sink unit 141 of the switching controller 140. Due to this feedback loop, the secondary voltage Vs across the secondary winding Ls of the transformer 121 is regulated into an essentially constant value.
The soft start unit 149 prevents the development of an excessive stress of the entire circuit when a maximum energy is transferred to the secondary winding Ls during the initial operation of the switching mode power supply 100. To carry out this functionality, the soft start unit 149 gradually increases the voltage that is applied to the PWM comparator 143.
FIG. 2 illustrates an excessive switching current generated during the initial operation (i.e., the Soft Start) of the conventional switching mode power supply 100. During the initial operation the switching device 147 is in the on state at every falling edge of the oscillator clock. During the initial operation, the soft start voltage Vsoft is lower than the MOSFET sensing voltage Vsense. PWM comparator 143 compares these voltages and PWM unit 142 turns off the switching device 147. This off state is changed to an on state at the falling edge of the oscillator clock, from oscillator 144. In the following, the term “minimum turn-on time” means the time it takes to change the state of the switching device 147 from off to on.
In response to the change of its input voltage, the PWM unit 142 outputs a control voltage to turn the switching device 147 from the on state to the off state with a time delay. This time delay is due in part to a circuit delay associated with the signal propagation delay across the PWM unit 142 and to a delay caused by the LEB unit 145. Due to this time delay, the MOSFET sensing current (drain current) Idrain passing through the switching device 147 becomes excessive, as illustrated in FIG. 2. If the MOSFET sensing current becomes excessive, the voltage provided on the MOSFET becomes excessive over the inherent voltage limit of the MOSFET when the MOSFET is off. The MOSFET acting as the switching device is very likely to be broken down due to the excessive voltage over the inherent voltage limit of the MOSFET makes.
The above problems also occur when the output voltage set by a user at the secondary winding of the transformer 121 is high. Therefore, these problems are more severe in high voltage systems. In FIG. 2, Vsoft is a voltage from the soft start unit 147 and it represents an increasing voltage having discrete levels. Idrain represents a drain current in FIG. 1 and it has a curve shown in FIG. 2 as a result of comparison between Vsoft and Vsense.